Hi Erwan,
First of all, PRACTICE is good. If someone lives in the city and one day they have the option to practice on a concrete surface or not at all, practice in a less than ideal environment is better than no practice (as long as that environment isn’t overtly dangerous).
Secondly: “Does concrete create some sort of “insulation from Qi, or are the reasons purely mechanical ?”
A little of both really. Mechanically, concrete has no give, so it can be a bit more jarring and unforgiving on the joints. This would be particularly true on movements such as #5 in the D&T set. Energetically, concrete has a tendency to “leech” and absorb chi from the body a bit. Specific to your situation, practicing on your deck is fine.
The question that usually follows this one is “What about sinking my chi below my feet when I’m on a suspended surface or several floors up in a building?” Bruce has said that the field of the Earth is very strong, making such relatively small elevations inconsequential. You can still connect to the Earth in an airplane. That being said, ultimately, I find that it feels best to get my bare feet (or as close to it as possible) on the Earth itself whenever possible.